In recent years, a personal apparatus represented by a mobile phone has been equipped with a digital camera function as a standard function. In this situation, an increasing demand for a smaller camera module has been raised. In the meantime, the camera module is required to meet the heavy demands for more and more sophisticated functions such as an auto-focusing function and image stabilizing function. An actuator for driving the optical system is essential to implement the sophisticated functions of the camera module. An actuator for the sophisticated functions while meeting the requirements for more compact and lightweight configuration attracts attention.
In this situation, an actuator using a shape memory alloy is attracting attention. The shape memory alloy is typically represented by a titanium-nickel alloy, and can be defined as the alloy that, after having been subjected to deformation below a predetermined temperature, comes back to the original status due to martensite transformation by raising the temperature above that predetermined temperature. This property of the alloy is utilized to provide the performance of the actuator by heating.
When the aforementioned personal apparatus typically represented by a mobile phone is used for imaging operations, it is a common practice that the liquid crystal screen mounted on the apparatus is used as a camera finder. Thus, the framing is performed by viewing the preview image displayed on the liquid crystal screen when imaging operations are performed. To determine the framing, the focus of the preview image must be adjusted at a predetermined distance such as infinity.
However, in the conventional camera module, the auto-focusing function is not executed to focus the camera on a subject until the imaging is executed. Thus, when the preview image is displayed, the image focused on nowhere is displayed because the image taking lens is positioned at the position closer to the image pickup element (so-called over-infinite position) than infinity. This over-infinite position is provided to absorb the manufacturing errors and the fluctuation caused by temperature dependency in the focal distance of the image taking lens, and the manufacturing errors and the fluctuation caused by temperature dependency of the lens barrel.
To address these problems, a proposal has been made in a control apparatus using an actuator containing a shape memory alloy, taking a camera shutter unit and image stabilizing unit as examples. According to this proposal, the driven member is moved to a predetermined reference position by applying a standby current to the shape memory alloy to preheat it prior to the control of the driven member by the actuator (Unexamined Japanese Patent Application Publication No. 2001-263221). This technique can also be used to the aforementioned focus adjustment.